Photographic images are traditionally printed onto photographic paper using conventional film based optical printers. Currently, the photographic industry is converting to digital imaging. One step in the digital imaging process is to utilize images obtained from digital cameras or scanned film exposed in traditional photographic cameras to create digital image files that are then printed onto photographic paper. Towards this end, the current invention relates to the area of digital image printing of digital image files onto photographic paper.
The growth of the digital printing industry has led to multiple approaches to digital printing. One of the first methods of digital printing employed was the use of cathode ray tube (CRT) based printers. While such printers provide a means for digital printing, the technology has several known limitations. The first is the reduced resolution as determined by the limitation of the phosphor and electron beam. The resolution limitation is more severe when printing a large format at high resolution, such as 8 inch by 10 inch photographic prints with resolutions approaching 500 pixels/in.
CRT printers tend to be expensive which is a severe shortcoming in a cost sensitive market. Also, CRT printers are limited in the ability to provide sufficient red exposure to the media when operating at frame rates above 10,000 prints per hour.
Another commonly used approach to digital printing is the laser based engine as shown in U.S. Pat. No. 4,728,965. Such laser based systems are generally polygon based flying spot systems using red, green, and blue lasers. Unfortunately, as with CRT printers, laser based systems tend to be expensive. Specifically, the cost of blue and green lasers remains quite high. Additionally, currently available lasers are not always as compact as would be convenient. Another problem with laser based printing systems is that the photographic paper used for traditional photography is not directly usable in a color laser printer due to reciprocity failure. High intensity reciprocity failure is a phenomena by which photographic paper is less sensitive when exposed to high light intensity for a very short exposure time. Flying spot laser printers expose each of the pixels for a very short time, on the order of a fraction of a microsecond. Optical printing systems expose the paper for the duration of the whole frame time, which can be on the order of seconds. Thus, a special paper is required for laser printers.
A more contemporary approach uses a single spatial light modulator such as a Texas Instruments digital micromirror device (DMD) as shown in U.S. Pat. No. 5,061,049 or liquid crystal device (LCD) modulator to modulate an incoming optical beam. Spatial light modulators provide both significant advantages in cost, allow longer exposure times, and have been proposed for a variety of different printing systems from line printing systems such as the printer depicted in U.S. Pat. No. 5,521,748, to area printing systems such as the system described in U.S. Pat. No. 5,652,661.
The first approach using the Texas Instruments DMD shown in U.S. Pat. No. 5,461,411 offers advantages common to spatial light modulator printing such as longer exposure times using light emitting diodes as a source as shown in U.S. Pat. No. 5,504,514. However, this technology is very specific and not widely available. As a result, DMDs may be expensive and not easily scaleable to higher resolution. The currently available resolution is not sufficient for all printing needs. Furthermore, there is no steady path to increased resolution.
The second approach is to use a liquid crystal spatial light modulator. Liquid crystal modulators are a low cost solution for applications involving spatial light modulators. Several photographic printers using commonly available LCD technology have been proposed. Some examples of such systems are described in U.S. Pat. Nos. 5,652,661; 5,701,185; and 5,745,156. Most designs revolve around the use of a transmissive spatial light modulator such as depicted in U.S. Pat. Nos. 5,652,661 and 5,701,185. Until recently, most spatial light modulators have been designed for use in transmission. While such a method offers several advantages in ease of optical design for printing, there are several drawbacks to the use of conventional transmissive LCD technology. Transmissive spatial light modulators generally have reduced aperture ratios and the use of (thin film transistor) TFT on glass technology does not promote the pixel to pixel uniformity desired in many printing applications. Furthermore, in order to provide large numbers of pixels, many high resolution transmissive LCDs possess footprints of several inches. Such a large footprint can be unwielding when combined with a printlens. As a result, most LCD printers using transmissive technology are constrained to either low resolution or small print sizes. To print high resolution 8 in. by 10 in. images with at least 300 pixels per inch requires 2400 by 3000 pixels. Spatial light modulators with such resolutions are not readily available. Furthermore, each pixel must have a gray scale depth so as to be able to render a continuous tone print and do so uniformly over the frame size
Most of the activity in spatial light modulators has been directed at projection display. The projectors are optimized to provide maximum luminous flux to the screen with secondary emphasis placed on contrast and resolution. To achieve the goals of projection display, most optical designs use high intensity lamp light sources. Additionally, many projector designs use three spatial light modulators, one for each of the primary colors, such as the design proposed in U.S. Pat. No. 5,743,610. Three spatial light modulators are both expensive and cumbersome. For projectors using a single spatial light modulator, color sequential operation is required. To maintain the high luminosity in combination with the color sequential requirements, a rotating color filter wheel is employed. This is yet another moving, large part further complicating the system.
An object of the present invention is to overcome the above-mentioned drawbacks of digital image printing on photographic paper, namely cost, resolution, and reciprocity failure. The recent advent of high resolution reflective LCDs with high contrast (greater than 100:1), such as described in U.S. Pat. Nos. 5,325,137 and 5,805,274 has opened possibilities for printing that were previously unavailable. Specifically, the inventive printer is based on a reflective LCD spatial light modulator illuminated sequentially by red, green and blue, light emitting diodes (LEDs), and where the LCD spatial light modulator may be sub-apertured and dithered in two directions, and possibly three to increase the resolution. This method has been applied to transmissive LCD systems due to the already less than perfect fill factor. Incorporating dithering into a reflective LCD printing system would allow high resolution printing while maintaining a small footprint. Also, because of the naturally high fill factor present in many reflective LCD technologies, the dithering can be omitted with no detriment to the continuity of the printed image. While devices such as the TI micromirror can incorporate a secondary mask as shown in U.S. Pat. No. 5,754,217, the mask may be displaced from the device or at the very least add to the processing complexity of an already complex device. The use of a single LCD serves to significantly reduce the cost of the printing system. Furthermore, the use of an area spatial light modulator sets the exposure times at sufficient length to avoid or significantly reduce reciprocity failure.
The progress in the reflective LCD device field made in response to needs of the projection display industry have provided opportunities in printing applications. One aspect of the inventive design is that a LCD designed for projection display can be incorporated into the printing design with little or no modification to the LCD itself. By designing the exposure system and data path such that an existing projection display device requires little or no modification allows inexpensive incorporation of a commodity item into a print engine.
Of the reflective LCD technologies, the most suitable to this design (though not the only reflective LCD) is one which incorporates a small footprint with an integrated CMOS backplane. The compact size along with the uniformity of drive offered by such a device will translate into better image quality than other LCD technologies. There has been progress in the projection display industry towards incorporating a single reflective LCD (see U.S. Pat. No. 5,743,612), primarily because of the lower cost and weight of single device systems. Of the LCD technologies, it is the reflective LCD with the silicon backplane that can best achieve the high speeds required for color sequential operation. While this increased speed may not be as essential to printing as it is for projection display, the higher speeds can be utilized to incorporate additional gray scale and uniformity correction to printing systems.
Spatial light modulator printing systems can incorporate a variety o methods to achieve gray scale. Texas Instruments employs a time delayed integration system that works well with line arrays as shown in U.S. Pat. Nos. 5,721,622 and 5,461,410. While this method can provide adequate gray levels at a reasonable speed, line printing TDI methods can result in registration problems and soft images. Alternate methods have been proposed particularly around transmissive LCDs such as the design presented in U.S. Pat. No. 5,754,305, which can also be incorporated into reflective LCDs. However, if the LCD is sufficiently fast, the proposed printer can create gray scale in area images adequately without time delayed integration or analog operation.